Above threshold s-wave resonances illustrated by the 1/2${}^{+}$ states in ${}^9$Be and ${}^9$B

Abstract

We solve the persistent problem of the structure of the lowest $1/2^{+}$ resonance in ${}^9$Be which is important to bridge the A=8 gap in nucleosynthesis in stars. We show that the state is a genuine three-body resonance even though it decays entirely into neutron-${}^8$Be relative s-waves. The necessary barrier is created by "dynamical" evolution of the wave function as the short-distance $\alpha$-${}^5$He structure is changed into the large-distance n-${}^8$Be structure. This decay mechanism leads to a width about two times smaller than table values. The previous interpretations as a virtual state or a two-body resonance are incorrect. The isobaric analog 1/2${}^{+}$ state in ${}^9$B is found to have energy and width in the vicinity of 2.0 MeV and 1.5 MeV, respectively. We also predict another 1/2${}^{+}$ resonance in ${}^9$B with similar energy and width.

Comments: To be published in Physics Letters B